Rotary steerable systems (RSS) are devices that direct a downhole drill bit in a desired direction while the drill string is being rotated for the purpose of controlling the path that a well bore makes. The rotary steerable tools are generally programmed by an engineer or directional driller who transmits commands using surface equipment (typically using either pressure fluctuations in the mud column or variations in the drill string rotation) which the RSS tools understand and gradually steer the drill bit in the desired direction. In a rotary steerable system, the bottom hole assembly (“BHA”) trajectory is deflected while the drill string continues to rotate. Rotary steerable systems have a biasing mechanism to bias the drill string into a desired trajectory. The biasing mechanism may either be a push-the-bit type, which exerts a force on a drive-shaft by pushing off the formation, or a point-the-bit type, which changes the angle of the bit axis by directly pushing on a driveshaft. In one example of a push-the-bit RSS, a group of expandable thrust pads extend laterally from the BHA to thrust and bias the drill string into a desired trajectory. For this to occur while the drill string is rotated, the expandable thrust pads extend from what is known as a geostationary portion of the drilling assembly. Geostationary components of the RSS are rotated at a roughly equal but opposite direction as the drill string, so that the geostationary components do not rotate relative to the formation while the remainder of the drill string is rotated. By maintaining the geostationary portion in a substantially consistent orientation, the operator at the surface may direct the remainder of the bottomhole assembly (BHA) into a desired trajectory relative to the position of the geostationary portion with the expandable thrusters.
To maintain a geostationary portion of the drill string with a net zero rotation relative to the formation, motion counter to the rotation of the drill string is generated resulting in a net zero rotation relative to the formation. Typically the geostationary section is created by a type of device that physically engages the formation to prevent rotation. These types of tools have an external geostationary housing that is mounted on bearings. In other cases drilling fluid flow is used to counter rotate the geostationary portion of the RSS. The drilling fluid flow is directed across a turbine or mud motor that turns in the target direction. Various devices, such as a continuously variable transmission, or electromagnetic clutches engaged to the counter rotating turbine are used to adjust speed of the counter rotating member. However, in all of these devices the input flow rate is based on other fluctuating drilling parameters and may not provide a consistent source of power for a counter rotating member of the geostationary portion of the RSS. Additionally, if the rotating motion of the drill string is not constant, which occurs during stick slip drilling conditions in a wellbore, the target tool face (or direction in which the drill string is being steered at a given time) cannot be maintained.
Like reference symbols in the various drawings indicate like elements.